Why is diffusion insufficient to meet the oxygen requirements of multi- cellular organisms like humans?

Explanation

In large multicellular organisms like human, the oxygen of air will have to traval large distances inside the human body to reach and every cell of the body. Now, diffusion is a very, very slow process, it will take a very long time to make oxygen available to all the body cells. Thus, diffusion is insufficient to meet the oxygen requirements of multicellular organisms like humans because the volume of human body is so big that oxygen (of air) cannot diffuse into all the internal cells of the human body quickly. So, when the size of the multicellular oranism is large, then respiratory pigments (such as haemoglobin) present in blood take up the oxygen from the air in the lungs and carry it quickly to all the body cells.